1. Field of the Invention
The present invention relates generally to telecommunication circuits. More specifically, the present invention relates to full-duplex speakerphone control circuits including a double-talk detector.
2. Description of the Related Art
Most modern speakerphones use half-duplex operation which switches transmission between the far-end talker and the near-end speakerphone user. System designers resort to half-duplex operation because the acoustic coupling between the speaker and microphone is much higher in speakerphones than in a handset where the coupling is mechanically suppressed.
Hands-free communication through a microphone and speaker typically results in acoustic feedback or howling because the loop gain of the system exceeds unity when audio amplitudes are adjusted to a reasonable level.
Howling is a condition occurring in full-duplex operation in which both the microphone and speaker are active at the same time so that, in conjunction with the reflection off the hybrid, a closed loop is created. The signal coupling between the speaker and the microphone causes feedback oscillation or howling when the coupling between the speaker and microphone is strong enough to increase the system closed loop gain above unity.
The solution to the howling problem has typically been half-duplex operation, in which either the transmit channel or the receive channel is active with both channels never active at the same time. Half-duplex operation prevents howling but diminishes the overall communication quality by clipping words and forcing the speaker at each end to wait for the speaker at the other end to stop talking.
In full-duplex conversation, both transmit and receive channels are active simultaneously. Telephone handsets allow full-duplex conversation quality.
A condition termed xe2x80x9cdouble-talkxe2x80x9d also creates difficulty in controlling a communication circuit such as a speakerphone. Double-talk occurs when both near-end and far-end speakers speak simultaneously and presents problems for both near-end echo and far-end echo. Echo in the communication circuit is often handled using an adaptive filter that determines an adaptive estimation function. The adaptive estimation function operates normally in a single-talk state, but in the double-talk state the adaptive estimation function is generally inaccurate because the adaptive filter attempts to compensate for an acoustic echo plus local speech superimposed on the echo. Therefore echo cancellation devices that include an adaptive filter for estimating the response of a room typically include a xe2x80x9cdouble-talkxe2x80x9d detection device that monitors a microphone signal to determine when a person speaks into the microphone.
One conventional double-talk detector declares double-talk when a microphone signal sample is greater than or equal to one-half the largest sample of a loudspeaker signal within a fixed number of the most recent samples in which the fixed number corresponds to the maximum delay for acoustic signals to pass from the loudspeaker to the microphone. If the microphone receives speech signals, the energy of the microphone signal is typically at least half the energy of the loudspeaker signal so that the conventional double-talk detector properly concludes that a speaker local to the near-end is speaking into the microphone and responds to the speech signals by disabling the adaptive filter from adjusting filter coefficient taps.
For a loudspeaker and a microphone are positioned far apart, the microphone receives little or no acoustic feedback from the loudspeaker, When a speaker speaks softly into the microphone, the energy of the soft voice component of the microphone signal alone is not greater than half the energy of the loudspeaker signal. The conventional double-talk detector incorrectly determines that no speaker is speaking into the microphone and responds by enabling the adaptive filter to adjust the filter taps. The filter begins adjusting the taps to eliminate the echo received at the microphone. By incorrectly determining that no speaker is speaking into the microphone, the device begins to cut off the near-end speech.
If the loudspeaker is positioned near the microphone, the energy of the microphone signal may exceed half the energy of the loudspeaker signal without regard to the occurrence of near-end speech. For example, if ambient background noise occurs in a room, such as noise generated by a fan, the microphone receives the noise and adds the noise to the acoustic feedback resulting from the near proximity of the microphone and loudspeaker. The microphone signal energy may exceed half of the energy of the loudspeaker signal even when the loudspeaker is the only source of sound in the room. The conventional double-talk detector incorrectly concludes that a speaker continuously speaks into the microphone, permanently disabling the adaptive filter from adjusting the adaptive filter coefficient taps.
In accordance with the present invention, a full-duplex communication circuit including a double-talk detector is provided for full-duplex, hands-free communication. The communication circuit includes a first signal path, a second signal path, and an echo canceller having an adaptive filter for accessing a first signal, determining a compensation signal from the first signal, and compensating a second signal using the compensation signal to form a compensated second signal. The circuit also includes an echo path presence and absence detector. The detector includes an Echo Return-Loss Enhancement (ERLE) calculator, a power estimator, a noise estimator, and an echo path presence and absence controller. The ERLE calculator is coupled to the echo canceller and has a logic for calculating an ERLE value. The power estimator is coupled to the first signal path and the second signal path, receives an uncompensated signal, determines a power level of the uncompensated signal, receives a compensated signal, and determines a power signal of the compensated signal. The noise estimator is coupled to the power estimator to receive the power signal of the compensated signal and generate a background noise signal. The echo path presence and absence controller is coupled to the ERLE calculator, the power estimator, and the noise estimator. The controller determines the presence or absence of a path based on current and past ERLE values, and current and past background noise estimates.
In accordance with another aspect of the present invention, a full-duplex communication circuit includes a first signal path for carrying first signals, and a second signal path for carrying second signals. An adaptive filter is coupled to the first signal path and the second signal path for accessing a first signal, determining a compensation signal from the first signal, and compensating a second signal using the compensating signal to form a compensated second signal. A suppressor is coupled to the first and second signal paths. The suppressor is selectively activated to attenuate the first signals in the first signal path and the second signals in the second signal path, but is otherwise inactive. An update controller is coupled to the adaptive filter for regularly updating filter coefficients of the adaptive filter and selectively inhibiting updating of the filter coefficients when the suppressor is activated.
In accordance with still another aspect of the present invention, a method of operating a full-duplex communication circuit is disclosed. First signals are carried on a first signal path from a first-end input terminal at a first-end to a second-end output terminal at a second-end. Second signals are carried on a second signal path from a second-end input terminal to a first-end output terminal. A signal is adaptively filtered by accessing a first signal, determining a compensation signal from the first signal, and compensating a second signal using the compensation signal to form a compensated second signal. The presence and absence of an echo path is detected by calculating an ERLE value, determining a power level of an uncompensated signal, determining power signal of a compensated signal, generating a background noise signal based on the power signal of the compensated signal, and determining the presence or absence of a path based on current and past ERLE values, and current and past background noise estimates.